Andy,

My conclusion then was, that if the microcontroller sent two bytes in rapid succession, and the UART receiver inside the Prolific chip missed the leading (SPACE-to-MARK) edge of startbit of the second byte; most probably as a consequence of faulty implemented UART reception stop (it most probably attempted to receive the WHOLE stopbit of the first byte, whereas it should end reception in the middle of stopbit and start waiting for the next byte's startbit). A tiny jitter or assymetry causes then the miss of the startbit leading edge.

You cannot do anything about it in FM - you can delay the echoed bytes, but you cannot put delay in between the bytes of answer to the ID class of requests (like :020000050000F9).

I seem to have a similar "cable" at home and am using it with a modem I am connected with at the moment without any problem, so I will now conduct some investigation similar to that one and get back with the result.

JW

PS. I was not using FM in that experiment beyond verifying the basic problem; but a general purpose terminal emulator (Bray's).